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Transactions of the American Fisheries Society

Abstract

Hourly fluctuations in flow from Glen Canyon Dam were increased in an attempt to limitthe population of nonnative rainbow trout (Oncorhynchus mykiss) in the Colorado River, Arizona,due to concerns about negative effects of nonnative trout on endangered native fishes. Controlledfloods have also been conducted to enhance native fish habitat. We estimated that rainbow troutincubation mortality rates resulting from greater fluctuations in flow were 23–49% (2003 and2004) compared with 5–11% under normal flow fluctuations (2006–2010). Effects of this mortalitywere apparent in redd excavations but were not seen in hatch date distributions or in theabundance of the age-0 population. Multiple lines of evidence indicated that a controlled flood inMarch 2008, which was intended to enhance native fish habitat, resulted in a large increase inearly survival rates of age-0 rainbow trout. Age-0 abundance in July 2008 was over fourfoldhigher than expected given the number of viable eggs that produced these fish. A hatch dateanalysis indicated that early survival rates were much higher for cohorts that hatched about 1month after the controlled flood ( April 15) relative to those that hatched before this date. Thecohorts that were fertilized after the flood were not exposed to high flows and emerged intobetter-quality habitat with elevated food availability. Interannual differences in age-0 rainbow troutgrowth based on otolith microstructure supported this hypothesis. It is likely that strongcompensation in survival rates shortly after emergence mitigated the impact of incubation lossescaused by increases in flow fluctuations. Control of nonnative fish populations will be mosteffective when additional mortality is applied to older life stages after the majority of densitydependentmortality has occurred. Our study highlights the need to rigorously assess instreamflow decisions through the evaluation of population-level responses.